General
In highlighting the art of helical scan recording, reference is made to FIG. 1 of this specification which illustrates some of the pertinent elements of a prior art head drum assembly. As can be seen from FIG. 1, forward moving (left to right) tape 10 passes by a conical guide-roller 20 and engages cylindrical drum 30 with the aid of roller 21. The conical shape of roller 20 causes tape 10 to slant downward as it passes by roller 21. At drum 30, the tape wraps half way around the drum, and with the aid of roller 41 passes by a conical guide-roller 40 which slants the moving tape upwards, thus nullifying the downward slant produced by roller 20. In this manner, tape 10 is made to wrap around drum 30 in a helical manner, emerging from drum 30 at a level different from that at which it entered.
Within drum 30 there is a rotating disc 300 containing a record/playback head 310 and a record/playback head 320 placed diagonally across from each other. Disc 300 rotates about an axis coincident with the axis of drum 30 and is horizontally aligned with the wrapped tape. Rollers 20 and 40 are so designed and positioned that the tape's lower edge in the neighborhood of roller 21 is at the same horizontal elevation as is the tape's upper edge in the neighborhood of roller 41; that elevation being the elevation at which disc 300 is rotating. Thus, for a stationary tape 10, it can be seen that a record/reproduce head on the circumference of disc 300 traverses a straight line upon tape 10 at a sharp angle with respect to the tape's edge. For a tape that is moving slowly with respect to the speed of rotation of disc 300, a straight line of an even sharper angle with respect to the tape's edge is produced.
FIG. 2 depicts some of the parallel tracks generated by heads 310 and 320. For example, tracks 11 and 13 may have been generated by head 310 and tracks 12 and 14 may have been generated by head 320. Conveniently, the tracks of helical VTRs (e.g., track 14) are long enough to contain the signals of a complete TV raster field. Since each TV field contains 262.5 horizontal lines, each track contains 263 blocks (e.g., blocks 141 and 142) with each block corresponding to a tape section which contains the signals of one line. Associated with the signal of each line there is a horizontal synchronization (H synch) pulse. This pulse is used to synchronize the TV receiver to the incoming signals. The H synch pulses are shown diagrammatically in FIG. 2 by line segments such as line segment 145 on track 14. Also shown in FIG. 2 is a control track 15 and an audio track 16. Audio track 16 contains the audio information corresponding to the video information within the slanted video tracks (e.g., track 14), and control track 15 contains various control signals.